The prevention or treatment of local or topical disease states or conditions of the skin has traditionally used simple non-occlusive delivery systems. These drug delivery systems usually include a volatile and/or non-volatile medium whereby a composition of the drug and medium is topically applied to the skin, generally in the vicinity of or directly on the area of skin to be treated. Such delivery systems usually take the form of emulsions, creams, ointments, foams, gels, liquids, sprays and aerosols. These delivery systems are generally used to treat skin inflammations, soft-tissue contusions, parasites, fungal and bacterial topical infection and topical analgesia. The limitation with this type of delivery system is that systemic drugs are generally not suitable for this type of administration. Some major problems with the current state of the art relate to a lack of efficacy of systemic drugs because of the low drug flux across the skin, as observed for drugs such as testosterone, amlodipine, fentanyl, buprenorphine and many others. Other drugs, such as glyceryl trinitrate, Nitrobid™ (a drug for the treatment of angina), are difficult to deliver by these systems due to the inability to adequately control the rate of drug delivery, or the requirement for a very large application area. Other problems with the poor dermal penetration of drugs is that the drug can be easily washed off or transferred to clothes, other surfaces or other animals.
The dermal delivery of drugs may represent the oldest form of drug delivery in human history. Resins and animal fats were probably used by humans in early times to treat damage to the skin resulting from injuries and burns. Such substances for local delivery of active substances remained largely unchanged until as late as this century. The concept of transdermal systemic drug delivery was first seriously advocated by Dr Alejandro Zaffaroni in U.S. Pat. Nos. 3,598,122, 3,731,683 and 3,797,494 from the early 1970s. Transdermal systemic drug delivery provides an effective method of achieving improved bioavailability for physiologically active substances where the drugs are poorly absorbed by traditional routes of delivery. It can also be used where oral dosing is poorly tolerated or not possible.
Transdermal formulations are however limited. For example polar drugs tend to penetrate the skin too slowly. Since most drugs are of a polar nature this limitation is significant, as is the fact that many drugs cause irritation at the site of topical application.
Two main methods are known for assisting the rate of penetration of drugs across the skin. The first is to increase the thermodynamic activity of the drug. The thermodynamic activity of a drug in a dermal formulation is proportional to the concentration of the drug and the selection of the vehicle. According to the laws of thermodynamics, the maximum activity of a drug is related to that of the pure drug crystal. The second method involves the use of compounds known as penetration enhancers to increase the permeability of the dermal surface and has generally proven to be more convenient and effective.
Since the early 1970s the main focus of transdermal systemic drug delivery has been, and still is, on transdermal patch devices. These patch devices are like bandages which are attached to the surface of intact skin for prolonged periods of time to allow a desired systemic delivery of a drug or other physiologically active agent. These transdermal patch devices occlude the skin and trap the drug, together with volatiles and vehicle excipients, between the skin and an outer impermeable backing membrane. The membrane prevents the evaporation or diffusion of vehicle excipients, volatiles and drug into an environment other than the target skin site. The prolonged length of time required for transfer of the drug and excipients from the patch into the skin can and often does result in local skin irritation. The irritation is caused by prolonged contact on the skin by the drug, volatiles, vehicle excipients, or the adhesive used to attach the patch device to the skin. The occlusive nature of the patch device also restricts the natural ability of the skin to “breathe”, increasing the risk of irritation. With added problems of complex and costly manufacturing processes for transdermal patch devices there is a need for improved transdermal drug delivery systems.
The rate of drug delivery across a dermal surface can be increased by dermal penetration enhancers. The problem with most known dermal penetration enhancers is that they are often toxic, irritating or allergenic. These enhancers tend to be proton accepting solvents such as dimethylsulfoxide and dimethyacetamide. More recently, 2-pyrrolidine, N,N-diethyl-m-toluamide (Deet), 1-dodecal-azacycloheptane-2-one (Azone®), N,N-dimethylformamide, N-methyl-2-pyrrolidine and calcium thioglycolate have been reported as effective enhancers. However, difficulties remain with such dermal enhancers because the problem of irritation at the site of application has not been overcome.
The most critical problem with these compounds however is their toxicity. If a compound when used as a dermal enhancer is toxic, irritating or allergenic, then that compound is unsuitable for application to the animal body. Dimethyl sulfoxide and dimethyl acetamide are not clinically acceptable for these reasons. Although Deet and Azone® have lower reported toxicities, their toxicity is still such that they are not widely used. It is possible that Azone® may be employed as a dermal penetration enhancer if the amount applied is sufficiently small so as not to be appreciably toxic, irritating or allergenic to the animal.
The thermodynamic activity of a drug can be increased by employing supersaturated systems which give rise to unusually high thermodynamic potentials [Coldman, et al., J. Pharm. Sci., 58(9), 119, 1969]. However, topical vehicles relying on supersaturation, have the major limitation of formulation instability, both prior to and during application to the skin. As such, they are of limited clinical value within a non-occlusive volatile:non-volatile delivery vehicle, because as soon as the formulation comes into contact with a person's clothing or the like, the drug often precipitates; hence the formulation is no longer supersaturated and any enhanced percutaneous absorption ceases.
Other workers such as Kondo, et al., [J. Pharmacobio-Dyn., 10, 743, 1987] who were using supersaturation to achieve enhanced transdermal drug delivery, have relied on the use of anti-nucleating polymers to stabilize the formulation. However, the applied drug formulations stabilised with polymers formed an appreciable surface mass on the skin which remained there over a prolonged duration of many hours, not a few minutes. So while Kondo advocated the use of a metered spray to deliver these formulations, in reality it would be impossible to obtain a non-occlusive delivery system with a short application time and still maintain a clinically useful transdermal penetration enhancement.
German patent application DE 4334553-A1 to Jenapharm GmbH discloses a pharmaceutical liquid system consisting of a drug (diclofenac), a lipophilic phase, a volatile component and appropriate antioxidants, preservatives or stabilisers. This system relies on supersaturation to increase the flux rate of dermal absorption. An application chamber is used to prevent accidental precipitation of the supersaturated drug delivery system over the application time of 150 minutes.
Japanese patent JP 61-268631 to Showa Denko KK discloses dermal penetration enhancers suitable for use with water-soluble drugs. The dermal penetration enhancers disclosed include 1-5 carbon fatty acid esters of para-aminobenzoic acid but their chemical structures are quite distinct from the compounds used in the present invention, and the physicochemical properties of the 1-5 carbon fatty acid esters of para-aminobenzoic acid are markedly different to those of the present invention. For example the octanol-water partition coefficients for all the 1-5 carbon fatty acid esters of para-aminobenzoic acid are at least 200 fold lower than those of the present invention. Also the preferred dermal penetration enhancer disclosed in JP 61-268631 is the 2 carbon fatty acid ester of para-aminobenzoic acid (or Benzocaine) which has an octanol-water partition coefficient which is more than 8000 fold lower than those of the present invention. Unlike those of the present invention, the preferred dermal penetration enhancer disclosed in JP 61-268631 has significant pharmacological properties in that it is a local anaesthetic, which has also been reported to cause irritant and allergic skin reactions. The compounds used in the present invention fulfil the ideal properties of a dermal penetration enhancer in that they are non-irritant and pharmacologically inert [Barry, B. W. Vehicle Effect: What Is an Enhancer? In: Topical Drug Bioavailability, Bioequivalence, and Penetration. Shah, V. P.; Maibach, H. I. Eds. Plenum Press: New York, 1993; pp 261-276.].
It was not surprising then to find that in previous studies [Feldmann, et al., Arch. Derm., 94, 649, 1996; Coldman, et al., J. Pharm. Sci., 58(9), 119, 1969; and Bhatt, et al., Int. J. Pharm., 50, 157, 1989] where low volumes of non-occlusive volatile:non-volatile vehicles had been applied to the skin, the extent of drug delivery was very limited. To date the only formulations that have been employed clinically are either for local therapies, such as topical minoxidil and topical non-steroidal anti-inflammatories, or for transdermal drug delivery of compounds which readily diffuse across the skin such as glyceryl trinitrate and isosorbide dinitrate. As the permeability coefficients of sex hormones, for example, are an order of magnitude lower than glyceryl trinitrate, a marked penetration enhancement effect would be needed to achieve clinically acceptable transdermal drug delivery.
It is desirable to have a clinically acceptable non-occlusive transdermal drug delivery system where the drug and penetration enhancer undergoes rapid partitioning into the skin to allow a convenient application time, leaving no residual formulation on the skin surface, and maintaining good substantivity within the skin. These characteristics can overcome problems such as a loss of drug penetration or possibly a transfer of the drug from the treated individual to another upon intimate contact, such as that observed for a testosterone ointment being used for a male patient, but which caused virilization in his female sexual partner [Delance, et al., Lancet, 1, 276,-1984].
It is an object of the present invention to overcome or at least alleviate one or more of the abovementioned disadvantages of the prior art systems.